The present invention generally relates to devices and methods for processing organic waste material which may be used in livestock operations such as dairies, feedlots, slaughterhouses, and food processing plants. More particularly, the present invention comprises a processing system for digesting animal excrement, food wastes and other organic waste material at atmospheric pressure resulting in combustible gas products and in materials which may be used for fertilizer or animal nutrients. A first embodiment of the disclosed apparatus allows a retention pond to be converted to a digester, thereby reducing the required initial capital investment. Among its other uses, a second embodiment of the disclosed apparatus may be used as a module in a system, where the system comprises a plurality of modules, allowing the operator to take one module out of operation without shutting down the entire waste processing system.
The use of anaerobic digesters for processing organic wastes is well known. There are two basic types of anaerobic digester, batch and continuous. In batch type digesters, the digester is loaded with organic materials, sealed for the digestion period, and then emptied when gas production stops. Once digestion is complete, the digested material, or effluent, may be removed from the digester and fresh organic material deposited. In continuous digesters, organic material is constantly fed into the digester. The material is pushed through the digester by either mechanical means or by the displacement action of new material pushing out effluent. There are three basic types of continuous digesters: vertical tank systems, horizontal or plug-flow systems and multiple tank systems.
Depending upon the particular embodiment, the disclosed apparatus comprises a horizontal digester which may be used either as a batch digester or as continuous digester. When utilized as a continuous digester, the apparatus comprises a hybrid between a plug-flow system and multiple tank system, because it may comprise multiple “tanks.” In plug flow digesters, raw organic waste, or feedstock, is introduced into one end of the digester and effluent is removed from the other end. Over the course of time, which may be up to a period of weeks, the waste is anaerobically digested by anaerobic bacteria within the feedstock. The organic feedstock is broken down and one group of bacteria convert the decomposed matter into organic acids. A second group of bacteria, methanogenic bacteria, convert the organic acids into volatile gases such as methane, carbon, monoxide, nitrogen, oxygen and hydrogen sulfide. Some of these gases, methane in particular, have value as fuel gas. The remaining organic material, the effluent, may contain useful components such as ammonia, nitrogen, potassium and phosphorous. The effluent is discharged from the digester and further processed as necessary for disposal or for further use, such as for fertilizer or livestock feed additive.
Depending upon the method used, the required retention times for effective digestion can be relatively long, requiring a substantial storage capacity as the organic material is processed. For example, U.S. Pat. No. 4,750,454, by Santina et al., discloses a digester vessel 100 to 110 feet in length, 22 feet wide by 12 feet deep, allowing for a retention time of 15 to 20 days. Long retention times typically require large volume digester vessels. Such large vessels can present several disadvantages to the operator. For example, large vessels require a large initial capital outlay for the vessel. A large footprint and foundation are generally required for such vessels. Moreover, large digester vessels make future expansion problematic. A relatively small-sized facility which has future expansion plans must either presently invest in a system having initial excess capacity, or purchase a smaller system and retrofit as the need arises. Large vessels can also present operational difficulties. If a facility utilizes a single large digester vessel, the entire system must be shut down while maintenance is performed on the vessel. For example, non-organic solids, such as sand and gravel, may collect at the bottom of the vessel, which require that the digester vessel be periodically taken out of service so that the accumulated non-organic solids may be removed.
Many efforts have been made to increase the efficiency of anaerobic digesters and/or increase the process flow rates through the vessel to reduce the required retention time and thus reduce the size of the digester vessel. It has been recognized that digestion rates are reduced it there is a lack of bacteria necessary for complete digestion. Therefore, many of the approaches to increasing digester efficiency concentrate on maximizing the amount of bacteria within the digester vessel. Because anaerobic bacteria thrive at temperatures of about 98 degrees F. (mesophilic) and 130 degrees F. (thermophilic), one of the most common ways of optimizing digester efficiency is to maintain the temperature in either the mesophilic or thermophilic range. While maintaining the digester in the thermophilic range increases the decomposition rate and thus the biogas production rate from those achievable in the mesophilic range, operating the digester in the thermophilic range makes the digester more sensitive to changes in feedstock composition and temperature. Digesters operating in the mesophilic range are less sensitive to operational upsets than those operating in the thermophilic range.
In addition to controlling digester temperature, other methods of increasing digester efficiency are known. One such method is to recycle large volumes of effluents high in suspended bacterial biomass. However, because the increased flow volume increases the total required retention time, this method does not necessarily result in smaller digester vessels. Another approach attempts to control the wavelength of the ambient light in the vessel so as to irradiate the interior of the vessel with light having an optimal wavelength for cultivation of the biological biomass. Another approach is to provide the digester vessel with microbe support structures which provide large surface areas for the biomass material to become attached.
The different approaches for increasing digester efficiency can require complex digester vessels with elaborate requirements for vessel geometry, internal vessel design, piping, pumping, and heating. In addition, taking a digester vessel out of service for maintenance typically requires taking the entire processing system offline while repairs are made to the digester vessel and/or to its component parts.